Examples of a fixed type constant velocity universal joint comprise a fixed type constant velocity universal joint of a Rzeppa type (BJ) (for example, Patent Literature 1) and a fixed type constant velocity universal joint of the undercut free type (UJ).
As illustrated in FIG. 17, the fixed type constant velocity universal joint of the Rzeppa type comprises: an outer joint member 3 having a spherical inner surface 1 in which a plurality of track grooves 2 are equiangularly formed along an axial direction; an inner joint member 6 having a spherical outer surface 4 in which a plurality of track grooves 5 paired with the track grooves 2 of the outer joint member 3 are equiangularly formed along the axial direction; a plurality of balls 7 for transmitting torque, which are interposed between the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6; and a cage 8 for holding the balls 7, which is interposed between the spherical inner surface 1 of the outer joint member 3 and the spherical outer surface 4 of the inner joint member 6. The cage 8 is provided with a plurality of circumferential window portions 9 for accommodating the balls 7.
The cage 8 is held in spherical contact with both the spherical inner surface of the outer joint member 3 and the spherical outer surface of the inner joint member 6. Curvature centers (O2 and O1) of ball-center trace lines of the track grooves 2 and 5 of the outer joint member 3 and the inner joint member 6 are positioned symmetrically with each other with respect to a joint center Oj. In other words, the curvature center O1 and the curvature center O2 are axially offset to each other in opposite directions and by equal distances from the joint center Oj. Specifically, the track groove 2 of the outer joint member 3 is offset from a joint center Oj to a joint opening side by a predetermined distance along a joint center axial line X, and the track groove 5 of the inner joint member 6 is offset from the joint center Oj to a joint deep side by the predetermined distance along the joint center axial line X. Here, the joint center axial line X refers to a straight line comprising an axial line of the outer joint member 3 and an axial line of the inner joint member 6 under a state in which an operating angle of the joint is 0°. A joint center plane refers to a plane which comprises centers of the torque transmitting balls 7 and is orthogonal to the joint center axial line. The joint center Oj refers to an intersection of the joint center plane and the joint center axial line.
Thus, ball tracks formed of the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6 each have a wedge-like shape gradually expanding from one axial side to another axial side. The balls 7 are accommodated respectively in the wedge-like ball tracks, and transmit torque between the outer joint member 3 and the inner joint member 6. The cage 8 is incorporated to hold all the balls 7 within a joint plane (plane perpendicular to a bisector of an operating angle).
Further, a structure of the fixed type constant velocity universal joints of the Rzeppa type, which has been used as a technical standard for many years, has six torque transmitting balls, and this structure has gained support from many users in terms of performance, reliability, and the like. In this context, the applicants of the present invention have developed and already suggested an eight-ball Rzeppa joint which has high efficiency and is drastically reduced in weight and compactified while securing strength, load capacity, and durability which are equivalent to or higher than those of the six-ball Rzeppa joint as the technical standard (for example, Patent Literature 1 below).
Next, as illustrated in FIG. 18, the fixed type constant velocity universal joint of the UJ type comprises: an outer joint member 13 having an inner surface 11 in which a plurality of track grooves 12 are equiangularly formed along an axial direction; an inner joint member 16 having an outer surface 14 in which a plurality of track grooves 15 paired with the track grooves 12 of the outer joint member 13 are equiangularly formed along the axial direction; a plurality of balls 17 for transmitting torque, which are interposed between the track grooves 12 of the outer joint member 13 and the track grooves 15 of the inner joint member 16; and a cage 18 for holding the balls 17, which is interposed between the inner surface 11 of the outer joint member 13 and the outer surface 14 of the inner joint member 16. The cage 18 is circumferentially provided with a plurality of window portions 19 for accommodating the balls 17.
In this case, each of the track grooves 12 of the outer joint member 13 is formed of a deep-side track groove 12a in which a circular arc portion is formed along a track-groove ball-center trace line, and an opening-side track groove 12b in which a straight portion parallel with an axial line of the outer joint member is formed along the track-groove ball-center trace line. A curvature center O2 of the deep-side track groove 12a is shifted in an axial direction from a joint center Oj to an opening side of the outer joint member 13. Further, each of the track grooves 15 of the inner joint member 16 is formed of a deep-side track groove 15a in which a straight portion parallel with an axial line of the inner joint member is formed along the track-groove ball-center trace line, and an opening-side track groove 15b in which a circular arc portion is formed along the track-groove ball-center trace line. A curvature center O1 of the opening-side track groove 15b is provided apart by an equal distance F in the axial direction from the joint center Oj to a deep side opposite to the curvature center O2 of the deep-side track groove 12a of the outer joint member 13.
As described above, in contrast to the Rzeppa type in which each of the tracks is formed into a circular arc shape as a whole, a track shape of the outer joint member 13 of the UJ type is undercut free, specifically, straight on the opening side. Thus, in the opening portion, the balls are positioned on an outer side than those in the BJ type. With this, a shaft (shaft to be fitted into the inner joint member) and the track grooves 12 of the outer joint member 13 interfere with each other at higher angles. As a result, an operating angle that can be formed in the UJ type is higher than that in the BJ type. Further, the track shape of the outer joint member 13 of the UJ type is straight on the opening side. Thus, a radial moving amount of the balls 17 increases in an outer direction, and in accordance therewith, it is necessary to set an outer diameter of the cage 18 for holding the balls 17 to be larger. Therefore, a spherical inner diameter of the outer joint member 13 increases.
However, in the UJ type, the inner surface (spherical inner surface) of the outer joint member 13 is large, and hence the circular arc track groove of the outer joint member 13 is offset to the opening side. As a result, a track depth on the deep side decreases. Therefore, when the spherical inner surface of the outer joint member 13 is increased as described above, the track groove depth on the deep side further decreases. Here, the track depth is expressed as a distance from a ball contact point to a spherical surface, the distance being measured at a position at which a contact ellipse of the ball, which moves in the track into the axial direction and directions of contact angles, comes closest to a spherical surface during one rotation based on results of analysis on a joint internal force in a rotational state.
Further, with regard to holding of the balls 7 and 17 respectively with the cages 8 and 18 and securing of the track depths, a ball diameter in the UJ type is set to be larger than that in the Rzeppa type of the same size. In addition, a pitch circle diameter PCD of the ball, and in accordance therewith, an outer diameter of the outer joint member are set to be larger.
The fixed type constant velocity universal joint of the UJ type illustrated in FIG. 18 has a cage offset shape advantageous in securing the track depth on the deep side of the outer joint member. Specifically, a center O4 of a spherical outer surface 18a of the cage 18 is offset toward an axial opening side by fc with respect to the joint center Oj, and a center O3 of a spherical inner surface 18b of the cage 18 is offset toward an axial deep side by fc. The cage offset of such a type is referred to as track direction cage offset.
In recent years, there has also been proposed an eight-ball joint of the UJ type, which has a smaller outer diameter than that of the six-ball type (Patent Literature 1). A ball diameter of the eight-ball joint of the UJ type is smaller than that of the six-ball type. Thus, an offset amount is set to be small so that a radial dimension (thickness) of the cage, which corresponds to the above-mentioned radial moving amount determined based on a PCR (length of a segment connecting a center of a circular arc of the track groove of the outer joint member or a center of a circular arc of the track groove of the inner joint member and a center of the ball to each other) and based on an offset amount, can be secured irrespective of the size and the number of balls. In addition, as illustrated in FIG. 18, the eight-ball joint of the UJ type employs the cage offset.
In this context, higher strength and durability at high angles are important factors in terms of further compactification of such an eight-ball joint of the UJ type.
By the way, conventionally, there have been proposed constant velocity joints of a six-ball Rzeppa type, in which a center of the track groove is offset to a position spaced apart from the joint center axial line toward a radially opposite side with respect to the track groove (Patent Literatures 2, 3, and 4).
According to Patent Literature 2, the track groove of the outer joint member is formed of an opening-side first guide groove about the joint center and a deep-side first guide groove about a point offset from the joint center toward a radially opposite side. Further, the track groove of the inner joint member is formed of a deep-side second guide groove about a point offset from the joint center toward the deep side along the joint center axial line and an opening-side second guide groove about a point offset from the center of the deep-side second guide groove further toward the radially opposite side.
With such a structure, a groove depth of the deep-side first guide groove of the outer joint member is large, and a thickness of the inner joint member is large at a part corresponding to the opening-side second guide groove of the inner joint member. Thus, even when the joint forms a high operating angle, the ball does not climb onto the deep-side first guide groove of the outer joint member, and hence does not chip an edge part of the deep-side first guide groove. In addition, the inner joint member is not damaged by load from the ball.
According to Patent Literature 3, the center of the track groove of the outer joint member and the center of the track groove of the inner joint member are respectively offset to positions spaced apart from a diametric plane (joint center plane) toward both axial sides by equal distances and spaced apart from the joint center axial line toward radially opposite sides by predetermined amounts. With such a structure, under a state in which the joint forms a maximum operating angle and the balls come extremely close to an inlet edge portion of the track groove of the outer joint member, a contact force of the ball and the track groove is reduced. As a result, the inlet edge portion of the track groove is prevented from being damaged.
According to Patent Literature 4, respective curvature centers of groove center lines of the track groove of the outer joint member and the track groove of the inner joint member are set to be decentered toward both sides with respect to the joint center plane and to be positioned on opposite sides beyond an axial center on a plane comprising the groove center lines and the axial center. With this, a maximum permissible angle of a joint angle can be increased, and strength is secured without involving an increase in outer diameter of the outer joint member.
Further, conventionally, there has been proposed a joint capable of increasing a maximum bending angle without influence on traveling characteristics or the like (Patent Literature 5). Specifically, according to Patent Literature 5, an intersection angle between a tangent of a trace curve and a joint rotational axial line monotonically increases from a point at which a maximum distance between a base of a rolling path and the joint rotational axial line is measured.